Automatic telephone system



C. E. LOMAX AUTOMATIC TELE-PHONE SYSTEM -Jn.s,1929. 1,698,138

l Original Filed Jan. i8, 1926 4 Sheets-Shea?I 1 T0 Sti.- ffdllfS 6:/0. liv/5, /6-10 [ITE-Eff Claret-u: EE. L Umax Jan. s, 1929. 1,698,138

C. E. LOMAX AUTOMATIC TELEPHONE SYSTEM originalrilea `1911.18. 1926 sheets-sheet 2 Clarence E. Lnmax nun Bunn x 8 om 9 e,

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Jan. 8, 1929.

C. E. LOMAX AUTOMATIC TELEPHONE lSYSTEM Original AFiled Jqp. 18, 1926 4 Sheets-Sheet 4 mmzwmukm rmzwm. mhim UPENZDHDW. NW

Patented Jan. 8, i929.

CLARENCE E. LoMAX, or OAK PARK, ILLINoIs, AssIeNoR, Bv M EsNE ASSIGNMENTS,

To AUTOMATIC ELECTRIC INC., oExCnIoAeo, ILLINOIS, A CORPORATION 0E DELA- WARE.

AUTOMATIC TELEPHONE SYSTEM.

Application filed January 18, 1926, Serial No. 81,900. Renewed May 24,' 1928.

This invention relates to automatic telephone systems in general, but is more parf ticularly concernedwith trunking arrangements between a main automatic exchange and a private branch exchange. The object of this invention is the provision of a new and improved trunking arrangement for the trunking between a main automatic f ex.- change and one or more private branch exchanges especially adapted tor use when several private branch exchanges are each accessible from the main automatic exchange by large number of' trunks.` Another object Yof the invention is the pro` vision ot new andV improved automatic switches especially adapted for use in conjunction with the new trunking arrangement, however not being limned to such use.

A well known ,trunking arrangement em-l ployed for trunking betweena main automatic exchange and` private l branch exchanges is that in which trunks from the banks of a group of rotary connectors at the automatic exchange terminate at the operators7 switchboards at the private branch exchanges, the trunks to eachprivate branch exchange being accessible in a certain level of the rotary connector banks. The rotary connectors in turn are accessible in a particular level 'of the banks of a certain group of selectors.

`When the number of P. B. Xfs (the a breviation P. B. X. will hereafter'be used for private branch exchange accessible rom the M. A. X. (the abbreviation M. A. X. will hereafter be used for main automatic exchange approaches ten and when the number of trunks to these P. B. X.s

approaches ten, it may become economical to use one-digit connectors for the rotary connectors. In each level, then, access is had to the trunks to onecertain P. B. X. In this case, the corresponding contacts in each level of all the rotaryvconnectors are multipled together, thus providing ten trunks to each P. B. X. accessible in the level assigned to that P. B. X. If a P. B. X. requires more than ten trunks to take care of its traliic, the number approachingtwenty-five trunks, the corresponding bank contacts of all the connectors are not multipled together. The connectors as regards 'this level, are sub-divided into' several groups, the corresponding contacts of theA connectors in eachy sub-group being multipled according to the Vwell known practice, thereby increasing the number of outgoing trunks. Y

According to the well known trunking aru rangement, each trunk'v from' connector banks, instead of beingxconnected Adirectly to an outgoing trunk to the P. B. X. in

question, may terminate in a rotary lineswitch. The banks of all of these line switches may then lbe multipled together, so-.that each rotary lineswitch has access to all the trunks to the P. VB. X. in question. For further explanation, a particular case may beconsidered: A certain P. B. X. may v require twenty-live incoming trunks from the main automatic exchange to carry the traiiic. Itmay be supposed that'in order to give a required trunk hunting eiiiciency j under the existing trafc conditions, onedigit connectors are provided and, as regards the level assigned to this P. B. X., are divided into four sub-groups, the corresponding bank contacts in each sub-group f under allpossibilities will be extended via ,i

an outgoing secondary line switch to a trunk tothe P. B. X., providing there is at least one of the twenty-five trunks to the P. B. X. idle. To give this elhciency of trunk selection in the present assumed case, however, forty` line switches are required, one for each of ten trunks from each of the four sub-groups of connectors.

A. feature of thisinvention, however, is

the disclosurekof a scheme whereby the i number of secondary line switches in a case, such as assumed, can be 'greatly reduced without lowering the trunk selecting eiiif ciency any significant amount. According to this invention yit is proposed, instead of connecting each of the ten trunks of' each subgroup of connectors as regards the level in question to a rotary line switch, to conneet only the last few trunks offeach subgroup to rotary line switches, andl connect Y tary line switches access to all the trunks Figs. l to 4, inclusive.k

to the P. B. X. yVith this new'arrangement in all but the-more busy times ot the day, vthe calls over each sup-group ot connectors would be trunked directly to theP. B. X., and onlyduring the more busy periods, when the number oit calls in `any one sub-group of connectors approached ten, calls would be trunked to the P. B. X. via rotary line switches. l Furthermore, il vthe tratlic load is kept reasonably well distributed between the 'differentsub-groups, to the extent, for instance, that when ten,v si1nulta neous callsexist in some one sub-group there will be substantiallyas many calls in each.

of the remaining sub-groups as there are direct trunks to the P. B. yX. lfrom each subgroup, vno sacrifice in trunk huntingetli- Vciencywill be made, the new arrangement then giving the same etiiciency as in thev case when all the trunks *from the connector banks are Vconnectedto rotary line switches.

This arrangement may be moie explicitly described and itsadvantagcs explained by referring to the accompanying drawings,

Figs. 1 and 2,;when Fig.` 2 is placed to the right of Fig. l with the corresponding lines in alignment, is atrunkingdiagram ot' part of the particular telephonefsystem partially described in vthe preceding paragraphs, which illustrates the new trunking arrangement lproposed according tothis invention. Figs. 3 and 4, when Fig, 4 is placed to the right of FiO. 3 withthe corresponding lines in alignment, show by means ot the usual circuit diagrams the various automatic switches and the circuits at a P. B. X. through which a call may be extended from aM. A. X. to aP. B. X.l

Referring now tothe trunking diagram, Figs. .l and 2,. the assumption is made that the P. B. X. referred to previously as well as several other P. B. Xfs are accessible via trunks in one level of a Vcertain group ot selectors. This group of selectors is divided into twenty-subgroups. This division is cliagrammatically indicated at the extreme left of Fig. l, only seven of the twenty subgroups being shown. The lirst live trunks from each of the twenty sub-groups of selectorsjare connected directly to one-digit connectors making a totaler oneedigit connectorsaccessible directly `from selectors. The last tive trunks of each sub-group. of selectors terminate in v plunger type line switches through f an intermediate distributing Jframe designated IDF, thus utilizing aA total of 100 plunger line switches. The one digitconnectors are mechanically of the well known Strowger step-by-step type, and are responsive to one series of impulses to `raise their wipers opposite the corresponding level of bank contacts, and performl an automa-tic rotary operation to select an idle trunk in that level. The plunger' line switches also similar mechanically to the Vwell known Keith type of line switch, ditering in circuit arrangements will be ydescribed fully in later descriptions.

A total of 140 one-digit connectors have been provided to carry the tratlic to ythe several P. B. X.s in this telephone system.

With 100 of the one-digit connectors accessible directly trom, selectors, the remaining forty connectors are made accessible via plunger line switches. Since the plunger type line switch has a capacity oi only ten outgoing trunks, and since forty connectors are accessible via plunger line switches, the total of 100 line switches are divided into four groups of twenty-five line switches each, each group having access to ten oncdigitvconnectors. The usual crossconnecty ing scheme may be carried out at the IDFv so that in each ot' the twenty sub-groups of selectors, each oit the five outgoing trunks has access to a different group ot ten onedigit connectors as vfar as possible.v n A Only the level of one-digit connectors assigned to the-P. B. X. to be considered is shown. As described previously, 4it has been assumed that four groups ot' ten trunks each are required from this connector'bank level to carry the traiiic via twenty-five outgoing trunks to the P. B. VX. being considered.

Therefore, the total oitllO connectors as re-V gards this levelis divided into four groups ol' thirty-five connectors each, twentynfive laccessible directly from selectors, and ten from plunger line 'switches constituting one' group. The corresponding bank contacts of the connectors of each group lare multiply connected. As previously mentioned, v the bank multipling o't the connectors may differ in the Various levels, but since there are four groups of plunger line switches andV further since there are` four groups Otten trunks each outgoing from the one-digit connectors from the level under consideration, the connectors themselves have been shown in tour groups, thereby simplifying-the drawings.

In the case illustrated, the first six trunks from the connector' banks of each group of connectors are connected directly to` the outgoing trunks to the P. B. X. while thelast four trunks of each group of connectors are each connected to a rotary lineswitch. The rotary line switches are mechanically ofthe well known type of 1 rotary step-by-'step switch having twenty-tive sets o bank con` tacts, and in this case a set of busy contacts,

and which, at the termination of each call, rotates back to normal or its home position. The circuit detailsl of these rotary line switches form part of this invention and will be fully described in later descriptions. l

A graded multiple scheme betweenY the llU trunks from the four groups of connectors connected directly to the outgoing trunks has also been carried out as shown in the illustration. The first and lsecond trunks of each group are each connected directly to an outgoingtrunk to the P. B. X. in question; the third trunk and the fourth trunk of the first and 'second group and of the third .and fourth group are multipled together respectively, Vand connected to separate out-going trunks; while t-he fifth trunk and 'the siXth trunk of thev four groups are multipled together and connected to separate outgoing trunks.

To prevent the outgoing trunks connected directly to trunks from the connectors from being seized by the rotary line switches eX- cept under eXtreme tratlic conditions, the rotary yline switches being of the home position type, the said trunks are made last choice to the line switches.

The kcorresponding bank contacts of the four rotary line switches terminating the four trunks from a group of connectors are multipled together and connected tothe out-- going trunks. The niultipling between the four groups of rotary line switches is clearly indicated in the drawing and need not be further discussed. v

The trunkingscheme between the selectors and the one-digit connectors, i. e., the connecting of the first several trunks directly to connectors and the remaining trunks to line switches having access to other connectors, is not new, but is shown in this case to illustrate how this scheme can be used in a system such as assumed, and also to compare this scheme with the new trunking scheme betweentrunks from the connector banks and the outgoing trunks, i. e., the connecting of the rst several trunks from the connector banks directly to outgoing trunks and the remaining trunks Afrom the connector banks to rotary line switches having access to all the trunks to the P. B. X. including .those accessible directly from the connectors.

Referring now` to thev circuit diagrams, Figs. 3 and 4f, at the extreme left of Fig. 3 areshown three rows of bank cont-acts of a certain'level of the first sub-group of selectors, corresponding to the tenl terminals, selector sub-group 1, shown in Fig. 1. The trunk comprising conductors 5', 6 and 7 accessible at the first set of bank contacts 2, 3 and 4, (designated V in Fig. 1) terminates in the one-digitlconneotor C which is one of the 25 connectors of group 1 A shown in Fig. 1. The trunk comprising conduct-ors 5, 6 and 7 accessible at the sixth set of bank contacts 2, 3 and 4:, (designated V in Fig. 1) terminates at the IDF where it is` connected to the trunk comprising conductors 8, 9 and 10, the latter trunk terminatingv fIhe line switches of group 1 in whosebanks access is had to theten one-digit connectors of group 1 ]3. Connector C accessible 4at the Ibank contacts 21-24, inclusive, of line switch PLS l is one of the one-digit connectors of .group 1 .Bn y

Only the level of bank contacts of the y nectors Cl and C (multipled with the other connectors of groups 1-A and 1-B) are connected directly to outgoingtrunks to the P. B. X ,vwhile the last four lsets .of contacts of connectors C and C" (multipled with the other connectors of groups l-A and 1-B) terminate in rotary line switches. Thus the first set of bank contacts 74, 7 5,

and 7 6 of connector C (designated X:L in

Fin'. 1) and the first set of` bank contacts 7 t, 75, and 7 6 of connector C (designated X in Fig. 1) are multiply connected to conductors 91', 92 and93 (designated Z in Fig. land y Fig.l 2), respectively. The conductors 91 and 93 are connected to the'outgoing trunk conductors 101 and 102 which terminate at the private branch exchange in jack J. vThe seventh set ofl bank contacts 7 7 78', and 79 of connector C (designated Y in Fig. 1.) and the seventh set of bank contacts 77 78, and 7 9 of connector C (designatedjY kin Fig. 1)A are multiply connected together to conductors 91,92, and 93 (designated Z in Fig.' 1 and Fig. 2),respectively, the latter three conductors yterminating in the rotary line switch RLS. The line switch VRLS is one of the four rotary line switches of group No.

1 shown in Fig. 2, in whose banks access is had to all the outgoing trunks to theP. B, X. Each ltrunk terminates in a jack such as jack J at the. P. B. X. Since the trunk compris.- ing conductors 101 and 102 is first choice to the connectors of group N011, it is made last choice to 'the rotary line switch, as vdescribed previously. y

The interconnecting. trunks from the M. A. X. to the P. B. X. maybe two-way trunks. In this case, each trunk will also terminate in a line switch at the M. A., X. rFliese line switches may also be of the rotary .type similar to the line switch RLS, preferably having no normal position, however, and the circuit differing in that-respect. As indicated in Fig. 4, conductors 912 and 932 connected to conductors 101 'and'102, respectively, and

private conductor 922 terminate in a rotary line switch RLS. c The operators position is provided with one or more cords such as cord O, havingv an answering plugP anda callingplug The circuit otthis cord is new and 4simplitied, being practically the same for both ends. Associated with vthe cord O is the telephone set TS and the ,calling device S. `The lines operatorv at the branch ,exchange mayl not always be able to remove the connecting plugs immediately after thesubscribers replace their receivers at the completion of a call. It is desirable, however, thatas many as possible'ot they switches at the automatic exchange be released 'immediately after the calling subscriber replaces his receiver, independent of the operator at the branch exchange. vIt all the switches associated with 'a lcertain connectionwere released immediatelyl upon the replacement oi'the receiver by the calling party with the operators plug still in the jack of the trunk used1 a needless operation of the line switch connected .to this trunk at the automatic exchange and a first selector would result by reason ot the bridge across this trunk in the operators cord. This needless operation is ordinarily prevented by employing release-by-last-party connectors of a type which do not release until the operator removes the plug from the jack ot the trunk used, butA which momentarily remove ground from the release conductor .as soon asthe calling party replaces his receiver to Vpermit the preceding switches to ,restore to normal. AThe reapplication of ground on the release conductorprevents al connector held operated from being seizec until after it is again released following the removal of the plug from the jack of. the trunk'used.

The introduction of the plunger line switches between'the selectors and the onedigit connectors creates a special problem. lVithout employing some special means of prevention, the plunger line switch associated with a connection would be released` lcontrolling master switch has made a complete oscillation, it is evident that the trunk to this plunger line switch in question might be seized in the establishment of a second connection and cause this plunger line switch to again connect to the connector which may still be in an operated position. The second attempted connectionin this case obviously would not be completed. f J

.The circuits of the one-digit connector and the plunger line switch used have been so designed as to eliminate this possibility, this constituting another Vfeature ofthisA invention. lWhen the subscriber at the M. A. X. replaces his receiver at the end of a call, the one digit connector used't'unctions to permit the release otl the preceding selector switches, but maintains itself and` the associated plunger line switch inan operated position until the P. B X. loperator removes the plug from the jack of the trunk used. `With both the one-digit connector and the plunger line switch held in an operated position, the

trunk terminating in that line switch will be marked busy, and no selector will seize this trunk when so marked. Assoon asthe operator removes the plugfrom the jack, the connector and the plunger line switch vare restored and the associated trunk is again available for other connections.

The objects of the invention as expressed above and the features of the invention, those mentioned and'othersnot specifically stated, are described in the following descriptions of the operation of the various switches through which a call may be extended under various conditions. For this purpose it is first assumed that a subscriber at the automatic exchange wishes to establish Aa connec` tion with the subscriber at substation A of the P. B. X. and that the tralic is heavy at the time. To initiate the call, the subscriber at the automatic exchange removes his receiver and dialsthe directory number assigned to the P. B. X. in question. In response to the removal of the receiver and the dialling of the irst one or more digits (the number` selector in a sub-group of selectors having access to the trunks to this P. B. X., which may be assumed to be the sub-group 1. In response to another digit dialled, this selector operates to elevate its wipers opposite the proper bank contacts and then rotates its wipers in search for an idle trunk. Since it has been assumedthat the trafic at this time is rather heavy, it may bev 1further assumed that the first live trunks accessible in the banks of this selector are busy and the first trunk idle at this time is the trunk accessible in the sixth set of bank contacts connected to the terminals 2, 3., and 4, inclusive, terminating the trunk comprising conductors 5, 6, and 7, shown inrFig. 3. Upon the seizure of this trunk by the selector, ground potential is placed upon the private bank contact connected to the terminal 3, closing a circuit which may be traced from ground supplied tothe terminal .3,Qtrunk/ conductor 6, jumper 9', conductor 9, through the pull down windingv 11 of the plunger line switch PLS, closed contacts 13 Cil to the main battery supplied to conductor 13" at the master switch (not shown). The pull down winding v11 energizes and causes the operation of the plunger arm 15. The operation of the plunger arm 15causes the separation of contacts 13 and closes contacts 14, thereby closing a holding circuit for the pull down Winding which now includes resistance 16. The operation of the plunger arm also forces the bank springs 17-20, inelusive, into engagement with the bank contacts 21-24, inclusive, respectively, it being assumedthat the plungers of the Vidle line switches of the group containing the line switch PLS have been standing vopposite this set of bank contacts. Ground potential is now supplied to the conductor 29 from terminal 3, via bank spring19 and bank contact 23, terminating in the master switch bank. This causes the master switch to operate and position the ramaining idle line switch plungers opposite the next idle trunk in the well known manner.

By the engagement of bank springs 18 and 2O with bank contacts 22 and 24, respectively, a circuit is closedwhich extends from the bridge at the calling subscribers substation' and the two talking conductors lextending through the various automatic switches to terminals 2 Vand 4, conductors 5 and 7, jumpers Sand 10,', conductors 8 and 10, bank springs 18 and 20, bank contacts 22 and 24, conductors 28 'and 30, normally closed springs controlledV by armatures 42 and 43, through the upper and lower windings of line relay 32 to battery and ground, respectively, ground being supplied to the lower winding at the closed springs 64. Line relay 32l operates and closes a circuit for release relay 33 at armature 45 and the working contact. Release relay 33 operates and prepares a zcircuit v:tor the vertical magnet 38, which includes the series relay 34, at

armature 48 and the working Contact. At. armature 47, ground is supplied to the release conductor 29 to guard the trunk connected to the terminals l2, 3, and 4, from being seized by'another switch, and alsoto maintain the previously operated switches duced by the dialling of the next digit.

In response to the impulses produced by thedialling of the next digit, line relay 32 deenergizes and again energizes a corre'- sponding number of times. Each 'tiinethe' armature 45 'drops back, a circuit is closed for .the vertical magnet 38. The purpose.

of the resistance 68 shunting armature 6l will be explained subsequently. In response to the operationofv the Vertical'magnet 38, e

the Wipers 71, 72,fand 73V are raised `,Step-by-` step opposite the levelcorrespondingto theV digit dialled. The seriesrelay 34 is also energized in series with the rotary magnet 38 and functions in conjunction with the springs 66 which are closed upon the first vertical step of the switch lwipers to'close the.-

through the winding of the stepping relay 5 36 to battery. Relay 36 operates and closes a temporary holding circuit lfor itself` at armature 55, Vand prepares a circuit for the rotary magnet 39 at armature 56. Relay34 l is slow acting and therefore remains en? ergized during the entire vertical movement.

At the end of the vertical movement, series relay 34deenergizesand closes'the circuit' for the rotary magnet 39 atthe resting contact of armature 5,0. Rotary magnet 39A operates to step thev wipers into engagement p with the first set of bank contacts in the selected level, and also opens the interrupter springs 67 thereby opening the yholding circuit for the stepping 'relay 36. Stepping relay 36 deenergizes and opens the circuit ofthe magnet 394 at armature 56. Rotary magnetl 39 thereupon deenergizes'and again leo closes the interrupter springs 67. It the e trunk terminating in the'v bank contactsA 7 4-7 6, inclusive, is busy, there will be ground potential on the private testcontact 7 5` and a circuit will again be closed for the stepping relay 36 from ground on test contact 75, wiper 72, armaturel l58 and the resting contact, closed interruptersprings 67, springs 66 throughthe winding of stepping relay 36 to battery. y Stepping relay 36 will again operate and againjclose the circuit'of the rotary magnet 39 at armature 56. R0# tary magnet 39 will operate to step.;the wipers 71-73, inclusive, into engagement with the second set of bank contacts. This alternate operation of thestepping'v magnet 36 and the rotary magnet 39 will continue until the wipers 7 17 3, inclusive, vhave been rotated into engagement .with the bank contacts terminating an idle trunk', @annidle trunk being represented by an ungrounded privatetest contact. 'No circuit will be then closed for the stepping relay 36 'when' the wiper 72 ismoved into engagement there."- vvith.

It maybe assumed that the trunks-tere" Y minating in the first six sets of bank cony tacts of the connector C are busy at this time and that the trunk terminating in the bank vcontacts 77-79, inclusive, is the lirst idle` trunk. Thus the wipers 71-78, inclu sive, will be rotated into engagement with the'bank contacts 77-79, inclusive, respectively; and will be arrested` in this position. Acircuit maynow be traced from ground at armature 46 andthe working contact, through the winding of the switching relay 37,interrupter springs 67 through the winding of the stepping relay 86 to battery.

The resistance of the winding of the switchingrelay=37 is comparatively high and does not allow sufficient current to pass through the 'winding to operate the Stepping relay 36.l Relay 87` operates, however, and prepares the signalling circuit at armature 57 and 62,

f places ground potential on the wiper 72 and on the bank contact 78 to make this trunk test busy. .to other connectors, removes the shunt Vfrom around the resistance 68 and prepares circuits at armatures `59 and I6() which will be further described Subsc-- quently. 1 i

' The placingpof ground on `the private test contact 7 8 also initiates the operation ot the rotary line-switch- RLS by completing a Vcircuit from ground on this bank Contact,

conductorV 92, Varmature 98 and the resting contact, through the winding olik theline relay 95 to battery. Relay 95 operates and prepares a circuit for the switching relay 94 at armature 97 and closes a circuit for thestepping magnet 96 at armature 98. If the trunk accessible at the first set of bank contactswith which the wipers 91-94, inclusive, 'arepnormally in engagement is busy, ground potential will be present on the jtest contact 100. `Assuming this tov be the l"4:0

vris also busy, stepping magnet96 will again functionV as described aboveV to step the wipers another' step. During this stepping operationswitching krelay 94 isV prevented from operating byreason of the ground potential von both ends of its winding. This stepping operation continues until the switch wipers 91-93, inclusive, are stepped into engagement with the contacts terminating an idle trunk line, this beingy indicatedby theabsence of ground potential on the test contact witliwhich thewiper 92 becomes engaged. `No circuit will then be closed for the stepping magnet 96 and the wipers will y Legame set of their bank contacts.

Vbe arrested in that position. Furthermore',

ground -will be removed from the junction point oit' the lower terminal of the winding of the switchingrelay 94and'the stepping magnet V96. Switching relay 94 willoper# ate, its'circuit being traceable from ground at vthe working contact and armature 58', wiper 72,.bank'contact 78, conductor 92, armature 97, and the working contact, through the winding of the switching relay 94, interrupter springs 99. `through the windling of the stepping magnet 96 to battery. vThe resistance of the winding of the switch` ing relay 94 is'suiiicientlyl high to prevent the operation of the stepping magnet 96 in series therewith. Switching relay v94, in

operating, connects the wipers 91 and 98 to conductors 91 and 98 at armatures 96 and 100, respectively; opens a point in the return stepping circuit, to be described later, at armature99; closes a locking circuit for itself at armature 98;-and places ground upon the bank contact with w 92 is engaged (and the contacts multiply connected thereto, including test contacts of the connectors having direct access to the outgoing trunk in question) Vat armature 97 and the vworking contact, thereby protecting this trunk from being seized by some other rotary line switch or by connector since the rotary lineswitch RLS has selected this trunk, finding it' idle, it is apparent that this trunk became idle during the time of operation of the connector C and the rotary lineswitch RLS.,` ,Thus another vfeature 'oi this invention becomes apparent, this beingVV that those trunks to theP. B. -X. accessible directly inthe banks'olv the. one digit connectors may; be testedtwice, once by the one digit connector associated lwith the call in progressand once by the associated rotary lineswitch which was selected by that connector, after nding the trunks tothe P. B. X. accessible-directly in its banks busy.

lVhen armatures 96 and 100 were operated, the signalling circuit was closedwhich may be traced from groundat the ringing generator G, contacts 14 closed periodically by the interrupter I, resting contact and armature 54, armature 62and the working contact, wiper 73, bank contact 79, conductor 93, working contact and armature 100, Wiper ich the wiper.

93, conductor 102, armature 106 and the resting contact, through the windingrof sig-v nalling relay 103, the condenser 108, conductor 101, wiper 91, armature 96 yand the working contact, conductor 91, bank contact 77, wiper V71, workingF contact and armature 57, arma-ture 51 and theresting contact, through the upper windingy o1' the ring cut-ofi' relay 25 to battery. Relayv 103 is energized over this circuit and closes a circuit 'for the signal lamp L at armature v105. By Areason of a mechanicalV interlocking mechanism between the armatures of relays 103 and 104, relay 103 remains operated after its first energizetion untilit is unlocked by the operationy of relay 104.

In response to the lighting of the lamp L at the privatevbranch exchange the operator inserts the answeringplugo an idle cord, such as the plug P ot the cord O, into the jack J of the'calling trunk. Forcing the plug P into the jack J closes the springsV 107 completing a circuit for the switching relay 104. As a result, relay 104 operates its armature 106, thereby disconnecting "the signalling relay 103 and closing, a direct current circuit for therinq cut-oil'l vrelay 35 which is the same'as the circuit previously traced through the upper winding of relay 35 eX- cept that it now includes armature 106 and the working contact, tip and ring of jack J,

` tip and ring of plug P, normally closed conl tacts 130 and 131, armature 124 and the resting contact, and impedance coil 122. In operating,`relay 104also unlocks the armature of signaling relay 103,thus allowing this relay to fall back and open the circuit of the lamp L. Ring 'cut-off relay 35 operates over the previously traced circuit disconnecting the signalling'v circuit and extending the talking connection to the operators cord circuitat armatures 51v and 54. A locking circuit is also closed for the ring cut/ofiQ relay 35 from ground at armature 46 and the working contact, armature 53and the working contact, through the lower winding of the ring .cut-oit relay 35 to battery. Armature 52 is alsov actuated, but has no utility at this time. The operation 'of relay 35 also causes the closure of a circuit for the back bridgey relay 31 at armatures 51 and 54 and the working contacts, the lower winding of relay 31 now ditional ground supply will be described subse uently. l f Y f 'ter inserting the plugv P intothe jack J, the operator throws the listening key K to the left which is the listening` position, closing spring 134 and spring 135,.thereby bridging the telephone sets TS across the talking conductors.' She then requests the'number of the subscriber withi which connection is desired, and, assuming that thisnumber is that of the substation A, the operatorinserts the calling1 plug P into the jack J individual to the line of' the substation A, and then` throws the key K momentarily to the right whichisthe ringing position, closing springs 136 and springs 137. rlhe forcing oit the plug P into` the jack AJ separates springs 151 and springs 152-, thereby removing the normal bridge including the drop signal 150. from across the line conductors-161V and 162. A circuitgisalso closed for relay from ground by way of the sleeve of jack J, sleeve of plug P through the winding of relay 120 to battery. .Relay 120 operatesA and opens the normal bridge through the impedance 122 across the answering end ofthe cord O and substitutes therefore the ground and battery bridge through the battery :teed'relay 12 1"through which talking current is to be supplied for the transmitter ofthe called substation. a ln, operating, relay 120 also closes a circuit for the supervisory lamp L3, at varlnature .126.

The bells at the substation A ring responsive to the ringing current supplied by the generator R. To answer, vthe subscriber at the substationA removes his receiver thereby completing aV circuit through thereceiver and the transmitter ot the substation A, over line conductors 162 and 161, tip and ring of .the jack J, tip and ring of lplug P, springs 134 and 135', -key K having been restored, springs'130 and 131', armatures 124 and and their working contacts, through the upper and lower winding ofthe relay 121 to ground and battery, respectively. Relay .'121 operates vand opens the circuit of theA supervisory lamp L3 at armature 127. The extinguishing of the lamp L3 indicates .to the operator that the-called subscriber hasan-' swered'.- The calling and calledsubscribers may now hold conversation. e At. the termination of the conversation both subscribers will replace their receivers. Replacing therecevcr at the called substaf "ion A opens the circuit of the relay 121. 1n deenergizing, relay 121 again closes the circuit of the supervisory lamp L3 at arma-- ture 127. The relighting of the lamp L3 indicates to the'operator that the called sub- Scrib'er'has replaced his receiver.

lWhen the subscriber at the calling substation replaces his receiver, the bridge across the talking conductorsy is opened,

4thereby opening the circuit of the line relay Vlo `381to battery.

iicient current to flow through the winding' of the verticalv magnet 38 to operate `the magnet. Series relay 34 is operated, how* ever, and, being slow acting, remains in an energized position a short time after the release relay 33 has restored its armatures.

Ground potential is removed from the con ductor 29 momentarily, thereby allowing the preceding .switchesto restore to normal in the well known manner. The pull-down winding 11 of the plunger line switch PLS is also deenergized during this time, but the plunger arm v is held in position by the energization of the holding winding 12 which is energized over the circuit from ground at armature 60 and the working contact,

`working contact and armature 52, armature 41 and the working contact, holding conn ductor 27, bank contact 21, bank spring 17 through the holding windingv 12 to battery. A short timeafter relay'SS releases its armatures, series relay 34 drops back and again placesground potentialon the conductor 29,

since the grounded conductor 27 is connected to conductor ,29 at `armature 59 and the working contact, Incidentally the pulldown winding' 11 is also energized again. It will be seen then that responsive to, the replacing of kthe receiver at the calling substation, the

automatic switches preceding'the connector C and the plunger line switch PLS are Vreleased by the momentary removal of ground ,-7 from the private conductors 29 and 9, but

that the connector C and the plunger line switch PLSv are held operated, and that the trunk line extending to this line switch is kmarked busy by virtue of the replacement Vof ground potential on the conductor 9 to prevent its being seized at this time. The plunger line'switch PLS and; the connector C will remain in this operated position as long as the plug P is left in the jack J Y The lighting of the lamp L3, however, in-

dicates to the operator that the subscribers have finished 'the conversation, whereupony she will remove the plug P and Pf from the jacks J and J, respectively, as4 soonv as is possible. Removing the plug P from the jack J again'replaces the normal bridge across the subscribers lineV conductors '161 and 162, including the drop signal 150, and opens the circuit of the sleeve relay The circuit of the lamp L3 is opened at armature 126 and the impedance 122 is vagain connectedV across the talking conductors ofv of the answering end of the cord O at arma-z ture 124. n

Removing the plug P from the jack J' opens the circuit for vthe back bridge relay 31 by removing .the bridge including im-k pedance 122 across the heavy talking conductors, and also opens the circuit of the switching relay 104 by permitting :the separation of springs 107. In 'de'energizing, relay 31 closes a circuitrfor the release magnet 40 at armature 44 and the resting contact, which circuit may be traced from ground by way of armature 45 and the resting contact, armature 48 and the resting contact, armature 44 and. the resting contact, spring 63 which were closed upon the first vertical y step of vthe Vconnector C, through the winding of the release magnet 40 to battery. Release magnet 40 operates and releases the switch wipers inthe well known manner. The restoring of armature 41 opens the holding circuit of the ring cut-oill relay 35 and the switching relay 37. The current supply connections to conductors 28 and 30 through the windings of relay 32 are again restored'to normal by the release of armatures 42 and 43. Relays 35 and 37,v deenergize and restore their armatures back to normal.

The restoring ofthe connector. C removes ground potential from the testjcontact 7 8, thereby opening the holding circuit for the switching relay 94 of the rotarylineswitch RLS. Relay 94 deenergizes and restores its armatures back to normal. V Since the wiper 94 is now in engagement with a bank contact other than its normal or home position contact, a circuit may be traced from ground, all the contacts engaged by wiper 94 except the home position contact ybeing multiply connected to ground, wiper 94, .resting contact and armature 99, resting conta-ct and armature 98', interrupter springs v 99', through the winding` of the stepping magnet 96 to battery. Stepping magnet 96V operates to rotate the wipers step-by-step until Vthe wipers 94 again engage the ungrounded or home position.V rPheautomatic .switches and the` various circuits that were associated with the completed connection are nowcrestored to normal. I'

Had allthe trunks accessible to the ,con-

nector C in the selected level been busy at trunks-busy position or eleventh Vposit-ion.y When the shaft carrying the switch wipers 71-73, inclusive, is rotated to'this position,

a cam attached to the, shaft functions to close the rotary off-normal springs 65, at the same time separating the springs 64.

and release the connection thus far set up in the usual manner. 1

Ii the 25 trunks accessible in the banks of the rotary lineswitch RLS had been busy when the switch wipers 91'-94', inclusive, were being rotated in search for an idle trunk, the wipers would have been rotated into engagement with the last set of bank contacts which represents the all-trunksbusy position. The 25th contact engaged by the wiper 92' is a dead'contact. When the wipers are rotated to this point noi-circuit is closed for the stepping magnet 96' and the wipers will therefore be arrestedin this position. traceable from the. busy machine B', the wiper 91', armature 96 and the working contact, conductor 91, bank contact 77, wiper 71, working contact and armature 57, armature 51 and the resting contact through the upper winding of the ring cut-oli relay to battery. A busy tone current induced in the lower winding of the ring cut-oil relay 35 and i'iows to the talking conductor through the lower winding of the back bridge relay 31. Upon hearing the busy tone the calling subscriber will replace his receiver and try to establish the connection a little later.

Should a subscriber in the automatic exchange wish to establish a connection with the subscriber in the private branch exchange at ai time when the telephone trai'ic to the private 'exchange is rather light, the connection in all probability would be extended over a more direct route, exclusive of a primary lineswitch or a rotary lineswitch. A brief description of the establishment of such a connection may now be given. Again assuming that a subscriber in the main automatic exchange wishes to establish a connection with the subscriber in the private branch exchange such as the subscriber at substation A, he will remove his receiver and dial the number reserved for the private branch exchange and again instruct the answering operator accordingly. In response to digitsdialled, the'connection will be extended through various switches to an idle selector which, as previously assumed, may be one contained in the sub-group No. 1. This selector operates and raises its wipers opposite the proper level of bank contacts and then rotates to hunt an idle trunk line. It may be assumed that the trunk line accessible in the iirst set of bank contacts is idle this time, this trunk being the one connected to the terminals 2', 3', and 4', which are connected directly to the onedigit connector C' and that this is the trunk A busy tone circuit is now selected. A connection is then completed over the talking loop, bank contacts 2' and 4', conductors 5' and 7', normally closed contacts controlledby armatures 42' and 43', through the upper and lower windings of the line relay 32' to battery and ground,

respectively. The connector C' is the same as the connector C in all circuit details, f

with the'exception that the conductor, such as 27, has been disconnected froml the work-'- ing contact of armature 41, this conductor having utility only when such a connector` is accessible from a plunger line switch. The connector C' will operatey responsive to the last digit to be dialled by the calling subscriber to raise its wipers opposite the proper level of bank contacts and then automatically rotate them in search for an idle trunk to the P. B. X. As previously stated, in the banks, of the connector C' access is also had to trunks extending directly tothe P. B. X and to trunks terminating in rotary line switches, as is the case for connector C. It may now be assumed thatv the trunk terminating in the first set of vbank contacts is idle. Wipers 71', 72", and 73', will there- :tore be arrested in engagement with bank i contacts 74', 75', and 76', respectively.`

llVhen this takes place, switching relay 37' operates Vin the same manner as did the switching relay 37, this time, however, completing a circuit for the signalling relay 103 immediately by the operation of its armatures 57' and 62'. This circuit may be traced from the signalling generator G, contacts 14' closed intermittently by the interrupter I, resting contact and armature 54', armature 62' and the working contact, wiper 73', bank contact 76', conductor 93', trunk conductor 102, armature 106 and the resting contact, through the winding of the signalling relay 103, condenser 108, trunk conductor 101, conductor 91', bank contact 74', wiper 71' working contact 'and armature 57 armature 51" and the resting contact, through the upper winding of the ring cutoii relay 35' to battery. Signalling relay 103 operates and lights the lamp L1' to signal the operator. The other operations connected with the setting up of the connection to the subscriber at substation A are the same as those previously described.

Replacing the receiver at the called substation A, at the termination of the conversation again lights the lamp L3. The replacing of the receiver at the callingsubstation causesy the release of the line relay 32' which results in the momentary energization of series relay 34' 'and causes the momentary removal of ground from the private conductor 6' at armature 49' since relay 34' remains operated a short timeafter described. -After the release of relay 34',

ground is again supplied to the conductor 6' ment of another connection until the operal torhas again removed the plug P from the aio jack J. The momentary removal of ground from the holding' conductor 6 is for the pur'- pose of allowing the preceding switches to release.

The featureof the cord circuit O is privmarily its simplicity. The circuits of the two-sections of thecord are practically identical. The answering end of the cord O may be used to answer calls from either the main'A automatic exchange or from a private branch exchange subscriber. To answer a call from a branch exchange subscriber, the operator inserts the plug P into the ack of the calling line. Relays 120 and 121 opeiate in the saine manner as did relays 122 and 121 described above in the description of a call from a main automatic subscriber to the subscriber at substation A. The saine supervision is given by means of lamp L2 as was described for lamp L3.

Vith thisV cord`the operator at the private branch exchange is also able to set up a tail-to-tail connection; that is, the private branch operator may set up a call to two parties' and place them in talking communication with each other. VFor instance, a

` subscriber at a branch exchange substation,

set of impulses.

such as the subscriber at substation A, may call the branch operator and instruct her to pla-ce him in connection with some certain subscriber at the main automatic exchange asl soon as this desired subscriber is available. The operator will then insert the plug of an idle cord such as the plug P, into the jack of an idle Vtrunk extending to the main automatic exchange. It may be assumed that the trunk comprising conductors 101 and 102 is idle at this time and that the operator inserts the plug P into the jack J. Relay 104 operates from ground at the springs 107, closed by the insertion of plug P into the jack J and closes a bridge across vthe conductors 101 and 102 at armature 106, this bridge including armature 124 and the resting contact and the impedance coil 122. The closure of this bridge across the conductors 101 and'102 causes the initiation of the operation of the line switch RLS attlie main automatic exchange terminating the conductors 912-932, inclusive, in the well known manner. The line switch RLS operates toselect a trunk to an idle rstselector. When such a trunk is found, the selector is prepared to receive the Ground potential is also placed on the conductor 922, thereby guarding this trunk from being seized by a connector' such as the connector C,"or by a rotary lineswitch, such as the rotary line switch YRLS at the automatic exchange.

After inserting the plug P into thexjack J the operator operates the lie-y K to the right which is the dialling position, thereby closing the springs 132 and disconnecting points in the talking conductors of the cord circuit by separating springs 130 and springs 181. The operator then manipulates the calling Idevice S, dialling the number of the desired party in the usual inanner. 'y

When the connection with the subscriber at the automat-ic exchange has been conipleted, the operator inserts the calling plug of the cord in use, this being the plug P in this instance, into the jack J andr signals the subscriber at substation A in an ordinary manner. The two` subscribers will be -iii talking communication as soon as the subscriber at substation A removes his receiver. At the end of the conversation.disconnection will take place in a manner similar to that described in thepi'eviously described call.

In response to thereplacing of the receiver at the substation A, the supervisory lamp L3 is lightedindicating to the operator that the connection has been abandoned. In this case when the-receiver is replaced at the substation in the main automatic exchange, no switches are released, the switches in the automatic exchange being under the control of the operator.` As soon as the operator is notified of the discontinuation of the-call, she will remove the plugs Pand P from the jacks J and J. Removing the-plug P from the j ack J causes the release of the relay 104 which, at its armature 106, opens the bridge across the conductors 102 and 101 thereby causing the release of the line switch terminating the conductors 912-922, inclusive, and also causes the release ofthe automatic switches in they usual manner. Ground is also removed from the conductor 922 to againinark the trunk comprising conductors 101 and 102 idle. Removing the plug P from the jack J restores the liiiei`161 and 162 back to normal condition. g

The inventiomits features and operation, having been described, what is considered new and is desired to have protected by Letters Patent is specifically stated in the'V appended claims.

vWhat I claim is: 1.111 a telephone system, a calling line and a called linea selector switch, aline switch, a connector switch, kmeans for establishing a connection from said calling line to -said called line via said selector, said line switch, andfsaid connector,means responsive to tie replacement of the receiver `by the calling subscriber for releasing said selector, and means for maintaining said lineswitch and saidconnecto-r switch operated independent of said calling subscriber.

2. In a telephone system, a calling line and a called line, a selector switch, a line switch, a connector switch, interconnecting trunk lines each comprising two talking conductors and a private conductor, means for establishing a connection from said calling line to said called line via said selector, a trunk line, said line switch, a second trunk line, and said connector, means in said connector under the control of the calling subscriber for placing a characteristic potential on the private conductors of, said trunk lines to maintain all of said switches operated, means responsive to the replacement of the receiver by the calling subscriber for momentarily removing the characteristic potential from the private conductors of -both said trunk lines to cause the release of said selector switch, means for maintaining said line switch and said connector operated, and means lfor replacingv said potential on the private conductor ot vsaid second trunk line after said selector has been released.

3. In a telephone system, a calling line and a called line, an automatic. switch, a line switch, a connector, a t-runk line comprising two talking conductors and a private conductor, a second trunk line comprising two talking conductors, a holding conductor and a private conductor, means for establishing a connection from said calling line to said called line via said switch, said first trunk line, said line switch, said secondtrunk line, and said connector, means under the control of the calling subscriber for placing a characteristic potential on the private conductors of said trunk lines for maintaining said switches operated, and means under the control of the called subscriber for placing a chz-iracteristic potential on said holding conductor to pre-vent said line switch and said connector romreleasing when the characteristie potential is removed from said private conductors.

and said connector, means under the control of the calling subscriber for placing a characteristic potential on the private conductors of said trunk lines for maintaining' said switches operated, means under the control of the called subscriber for placing a cha-racteristic potential on said holding conductor to prevent said line switch and said connece tor from releasing when the characteristic potential is removed from said private conductors, andnieans under the control of the called subscriber for replacing said potential on the private conductor of said second trunk linefto prevent the establishment of change having access tosaid trunk lines,-

connector switches in vsaid automatic exchange having access to said hunt-ing switches at certain contacts vin their banks and having direct access to certain of said trunks at certain other contacts in their banks.

6. In a telephone system, a trunk line, a line switch having accessto said trunk line, a connect-or having access to said trunk line directly and indirectly through said line switch, means for-operating said connector to test said trunk line for the busy or idle condition, and for operating said connector to connect with said line switch providing said trunk line is busy when tested by said connector, and means for operating said line switch to test said trunk line a second time.

7 In a telephone system, a calling line, a called line, a selector, a line switch, a connector, means for lestablishing a connection from said calling line via said selector, said lineswitch, and said connector to said called line, andmeans in said connector controlled over the calling line for releasing said selec-` tor and for maintaining said line switch and said connector operated.

8. In a 'telephone system, a line, means i including three serially related switches for extending a connection from said line,l and means in the last switch for releasing the irst switch while maintaining the second switch operated.

9..In a telephone system, a line, a irst switch, other switches, means for extending a connection from said line via said first switch and said other switches in series, and means in one of said other switches controlled over said line for releasing said first switch and for maintaining said other switches operated. y

10. In a telephone system., a plurality of groups of connectors, a separate groupof line switches accessible to each group of connectors, a separate group'of trunks accessible to each group of connectors and accessible ,to all of said line switches, 'and another group of trunks accessible only to said line switches. n

11. In a telephone system, a plurality of groups of connectors, a separate group of trunks accessible to each group of connectors, another group of trunks accessible to some of the groups of connectors, another group Y nectors, a separate group of trunks accessible to each group of connectors and accessible to all of said line switches at corresponding contacts, and another group of trunks accessible only to said line switches at noncorre-A sponding contacts.

13. In a telephone system, a first switch, a second switch accessible' to said first switch, a trunk line accessible to both switches, and means responsive to the dia-lling of a digit for establishing afconnection from said first switch to said trunk line directly under certain conditions, and to said trunk line by way of said second switch under certain other conditions.

14. Ina telephone system, a first switch, a second switch accessible to the first switch, a first trunk line accessible to bot-h switches, a second trunk line accessible only to the second switch, and vmeans responsive to the dialling of a digit for establishing a connection from the irst switch to the first trunk line directly under certain conditions, and by way of the second switch under certain other conditions, and for establishing a connection from the first switch to the second trunk line by way of the second switch under certain other conditions, 1

means directively controlled-responsive to the dialling of the last digit of a telephone number for positioning the switch into selective relation with the row of contacts corresponding` to thedigit dialled, means responsive tothe positioning of said switch on any row of contacts for causing the switch to automatically select an idle contact in the selected row providing one is idle, means forgiving the calling party a busy signal providing allv the contacts in the selected row are busy, and battery feed bridges in said switch for feeding talking current to the calling and called lines.

16. In a telephone system, a trunk terminating in an automatic switch, said trunk comprising talking conductors, and two release conductors, means 'for seizing said trunk, means in said switch responsive to the seizure of said trunk for applying a characteristic potential to one of said release conductors, means for operating said switch to extend a call, means in said switch responsive to the answering of the call for applying a characteristic potential to the other release conductor, and means responsive to the replacement of the receiver' on the calling line for momentarily removing the potential on one of the release conductors.

In witness whereof, I hereunto subscribe my name this'ltth day of January, A. D.,

CLARENCE E. LOMAX. 

